The study, which brings bad news for Kylie Minogue, who turned 40 in May, pinpointed the milestone birthday as the moment women leave behind their carefree ways and start to plan for the future as a party of one.
According to the study that polled 1,000 women, one in 10 Britons blame putting their career first for their single status.
The research revealed that 32 per cent think they will feel "left on the shelf" once all of their friends settle down.
But 13 per cent believe it will happen when they start to lose their looks.
One in 20 even think it takes just a few comments from friends and family to start questioning whether they ever will find someone to love.
"Our research shows that 40 is the age when women turn from a happy-go-lucky singleton to spinster overnight," the Daily Express quoted a spokeswoman for shelving company Bigdug, which commissioned the study, as saying.
"Jennifer Aniston and Renee Zellweger both turn 40 next year so need to start dating soon to avoid the 'left on the shelf' tag. But with many females being so career focused, love lives are not the top priority any more," the rep said.